The present invention relates to a new and improved method of, and apparatus for, the teletransmission or remote transmission of signals.
The prior art is acquainted with remote control techniques where remote control commands are transmitted through the agency of alternating-current pulses or alternating-current pulse sequences. This is so for instance with remote control or signal transmission via high-current power supply networks, especially in the audio-frequency power line carrier control art. Thus, alternating-current pulses of a remote control frequency f.sub.s are superimposed upon an energy or power supply network having the network frequency f.sub.N. The transmission of a signal via a power supply network, for instance an intermediate voltage heavy-current network or a low voltage heavy-current network, in comparison to signal transmission via conventional communication channels, is associated with quite special problems.
While there is present the advantage of the wide dissemination or expanse of an energy supply network and thus the possibility of remotely controlling by means of the aforementioned signals a large number of apparatuses and current loads connected to such network, there is nonetheless present a high noise level of the high-current or heavy-current network as the communication transmission channel. The noise level of an alternating current-high-current power network is primarily constituted by disturbance frequencies which are certain harmonics of the network frequency f.sub.N. Oftentimes there however also occur transient disturbance voltages.
In view of the fact that in an alternating-current high-current network the network frequency f.sub.N is not constant as a function of time, it equally follows that the harmonics of the network frequency as the disturbance frequency are equally not constant as a function of time. With the selection of a suitable remote control frequency f.sub.S this condition must be taken into account. In consideration of the high noise level of the high-current network serving as the transmission channel it is additionally advantageous to select the band width of the receiver as narrow as such can be coordinated with the character of the information to be transmitted. Owing to this narrow band width it is therefore also necessary to undertake measures to insure that the throughpass frequency of the receiver, i.e., its response frequency always coincides as closely as possible with the actual transmitted remote control frequency f.sub.S. In consideration of the fluctuations of the network frequency f.sub.N it has been found to be advantageous to percentually change the remote control frequency f.sub.S transmitted via the high-current network to the same degree and in the same sense as the network frequency f.sub.N. The remote control frequency f.sub.S therefore is always in a fixed relationship to the network frequency f.sub.N. Due to these measures, during fluctuation of the network frequency, there is maintained the percentual difference or spacing of the remote control frequency f.sub.S from the neighboring harmonics of the network frequency f.sub.N, which occur as disturbance frequencies. Moreover, there are already known to the art techniques for the adjustment of the receiver response frequency to the fluctuating remote control frequency f.sub.S, and in this regard attention is directed to for instance Swiss Pat. No. 424,968. According to this technique the receiver response frequency is formed by a mixing process with the assistance of a pilot frequency f.sub.P, this pilot frequency f.sub.P being in the same relationship to the network frequency f.sub.N as the remote control frequency f.sub.S.
However, a drawback of this state-of-the-art technique is the dependency of the magnitude of the output signal of the receiver upon the phase position, which is variable and not predeterminable, between the received remote control signal and the pilot frequency f.sub.P generated at the receiver location, preferably from the network frequency.